Speed up rendering of RGB images

In my simple test case this made map rendering of RGB satellite image tiles
go down by ~50% from ~40ms per tile to ~20ms per tile (in debug version, ahem)

python/core/auto_generated/raster/qgsrasterblock.sip.in

@@ -166,6 +166,7 @@ returned value is undefined.
 :return: value *
 %End
 
+
     QRgb color( int row, int column ) const;
 %Docstring
 Read a single color
@@ -246,6 +247,7 @@ Set color on index (indexed line by line)
 :return: true on success *
 %End
 
+
     bool setIsNoData( int row, int column );
 %Docstring
 Set no data on pixel

src/core/raster/qgsmultibandcolorrenderer.cpp

@@ -225,9 +225,22 @@ QgsRasterBlock *QgsMultiBandColorRenderer::block( int bandNo, QgsRectangle  cons
     return outputBlock.release();
   }
 
+  QRgb *outputBlockColorData = outputBlock->colorData();
+
+  // faster data access to data for the common case that input data are coming from RGB image with 8-bit bands
+  bool hasByteRgb = ( redBlock->dataType() == Qgis::Byte && greenBlock->dataType() == Qgis::Byte && blueBlock->dataType() == Qgis::Byte );
+  const quint8 *redData = nullptr, *greenData = nullptr, *blueData = nullptr;
+  if ( hasByteRgb )
+  {
+    redData = redBlock->byteData();
+    greenData = greenBlock->byteData();
+    blueData = blueBlock->byteData();
+  }
+
   QRgb myDefaultColor = NODATA_COLOR;
 
-  for ( qgssize i = 0; i < ( qgssize )width * height; i++ )
+  qgssize count = ( qgssize )width * height;
+  for ( qgssize i = 0; i < count; i++ )
   {
     if ( fastDraw ) //fast rendering if no transparency, stretching, color inversion, etc.
     {
@@ -239,10 +252,17 @@ QgsRasterBlock *QgsMultiBandColorRenderer::block( int bandNo, QgsRectangle  cons
       }
       else
       {
-        int redVal = ( int )redBlock->value( i );
-        int greenVal = ( int )greenBlock->value( i );
-        int blueVal = ( int )blueBlock->value( i );
-        outputBlock->setColor( i, qRgba( redVal, greenVal, blueVal, 255 ) );
+        if ( hasByteRgb )
+        {
+          outputBlockColorData[i] = qRgb( redData[i], greenData[i], blueData[i] );
+        }
+        else
+        {
+          int redVal = ( int )redBlock->value( i );
+          int greenVal = ( int )greenBlock->value( i );
+          int blueVal = ( int )blueBlock->value( i );
+          outputBlockColorData[i] = qRgb( redVal, greenVal, blueVal );
+        }
       }
       continue;
     }

src/core/raster/qgsrasterblock.h

@@ -196,6 +196,20 @@ class CORE_EXPORT QgsRasterBlock
      *  \returns value */
     double value( qgssize index ) const;
 
+    /**
+     * Gives direct access to the raster block data.
+     * The data type of the block must be Qgis::Byte otherwise it returns null pointer.
+     * Useful for most efficient read access.
+     * \note not available in Python bindings
+     * \since QGIS 3.4
+     */
+    const quint8 *byteData() const SIP_SKIP
+    {
+      if ( mDataType != Qgis::Byte )
+        return nullptr;
+      return static_cast< const quint8 * >( mData );
+    }
+
     /**
      * \brief Read a single color
      *  \param row row index
@@ -332,6 +346,20 @@ class CORE_EXPORT QgsRasterBlock
       return true;
     }
 
+    /**
+     * Gives direct read/write access to the raster RGB data.
+     * The data type of the block must be Qgis::ARGB32 or Qgis::ARGB32_Premultiplied otherwise it returns null pointer.
+     * Useful for most efficient read/write access to RGB blocks.
+     * \note not available in Python bindings
+     * \since QGIS 3.4
+     */
+    QRgb *colorData() SIP_SKIP
+    {
+      if ( !mImage )
+        return nullptr;
+      return reinterpret_cast< QRgb * >( mImage->bits() );
+    }
+
     /**
      * \brief Set no data on pixel
      *  \param row row index